And all of that military hardware topped off with a warning from Russia's president one day after a U.S.-built missile-defense system went online in Romania.

"Until now, those taking such decisions have lived in calm, fairly well-off and in safety. Now, as these elements of ballistic missile defense are deployed, we are forced to think about how to neutralize emerging threats to the Russian Federation," Vladimir Putin told a meeting of top Russian defense and military industry officials on May 13. "All these are additional steps toward throwing the international security system off balance and unleashing a new arms race."

Even beyond the Kremlin, 25 years after the end of the Cold War and with Russia and Western powers squaring up over continuing conflicts in Ukraine and Syria, there are fears that Moscow and Washington are on the cusp of a new arms race -- nuclear, conventional, or both.

Russia has sent eye-catching signals about its weaponry in recent months: new cruise missiles fired from Caspian Sea naval ships at Syrian targets; the suspected deployment of short-range ballistic Iskander missiles to the Baltic Sea exclave of Kaliningrad; new ballistic-missile submarines going operational; and the Russian undersea fleet and long-range-bomber patrols approaching Cold-War tempos.

The United States has meanwhile ramped up its military operations in Eastern Europe and adjacent seas. The Pentagon is quadrupling its spending on European defense initiatives. Naval ships and U.S. aircraft are conducting more frequent surveillance patrols near Russia's borders.

An additional U.S. Army combat brigade is scheduled to start rotating into Europe and the top U.S. commander in Europe has suggested he would support a "permanently stationed armored brigade" on the continent.

"The thing with arms race dynamics [is that] no one has to intend to run an arms race for that dynamic to take over," said Jeffrey Lewis, an expert in nuclear nonproliferation at the Middlebury Institute International Studies in Monterey, California. "I worry about the increasing intensity of the deployments."

Gravity Bombs, Nuclear-Tipped Missiles

Then there are the nuclear arsenals. Strategic warhead and delivery-system counts in both countries have been more or less dropping, thanks to the 2010 New START treaty.

But both countries are at the same time modernizing other parts of their arsenals. The United States is moving forward with a multidecade, multibillion-dollar upgrade of its weapons, which includes the B61-12 nuclear gravity bomb, 180 of which are based in Europe, about which the Kremlin has already expressed displeasure. The U.S. administration is also moving forward with a controversial new nuclear-tipped cruise missile.

Russia is expected this year to flight-test a new super-heavy, silo-launched, intercontinental ballistic missile, or ICBM, capable of carrying up to 12 warheads and affectionately dubbed Satan-2, after its much-feared Soviet predecessor.

A breathless report by the Russian Defense Ministry TV channel Zvezda claimed the missile, scheduled for deployment by 2018, would be able to destroy the entire state of Texas.

"There's an arms race in the sense of, 'Hey, we can still keep up with what the U.S. is doing; the U.S. is building a fifth-generation fighter aircraft; we can build a fifth-generation fighter aircraft, but we're not going to buy 150 of them," said Dmitry Gorenberg, senior research scientist at the Virginia-based research group CNA, who specializes in the Russian military.

The prospect of a rekindled Cold War-style arms rivalry is in many ways a remarkable reversal from the situation early in U.S. President Barack Obama's first term, when Moscow and Washington tried to "reset" bilateral relations that had soured over issues like the 2008 war in Georgia and Russian opposition to the U.S.-led war in Iraq.

But now, with Putin asserting Russian power and influence in neighboring Ukraine and Syria, arms-control experts say, the chances of a new agreement to reduce arsenals further is slim to none.

A larger danger may be the fraying of existing ones, like the Intermediate-Range Nuclear Forces treaty, the 1987 treaty known as the INF that both Washington and Moscow have accused the other of violating.

Russian warhead counts under New START have also risen recently, prompting some concern, though experts say the fluctuation doesn't necessarily mean Russia will fail to meet a 2018 deadline for compliance.

"Unless a new arms-reduction agreement is reached in the near future, the shrinking of Russia's strategic nuclear arsenal that has characterized the past two decades will likely come to an end," arms scholars Hans Kristensen and Robert Norris wrote in an article published this month in the Bulletin Of The Atomic Scientists.

Russian officials have pointed to last week's activation in Romania of the missile-defense system, and specifically the launch system used to fire missile interceptors, as being in violation of the INF, something Putin pointedly raised in his comments. U.S. officials have countered that the system, which is similar to one used to launch Tomahawk cruise missiles aboard Navy ships, complies with treaty restrictions.

Last year, Russian TV aired purported plans for an underwater, nuclear-capable drone that would have the ability to shower a coastal area with radioactive fallout, making large regions uninhabitable for decades.

Soaring Spending

To be sure, there are politics holding back the push forward into a full-blown arms race. But more than anything, what may be constraining both are fiscal concerns.

Defense spending has soared under Putin, particularly since 2007, and was estimated at nearly 4.3 percent of gross domestic product in 2015. Since then, however, the country's economy has suffered due to low oil prices along with Western economic sanctions and retaliatory Russian bans, and the Kremlin is poised to cut its defense spending this year by 5 percent, the largest figure since Putin was elected president in 2000.

That has affected some notable weapons programs, including the planned revival of a railway-based ballistic-missile launch system, part of Moscow's effort to deepen the country's nuclear deterrent capabilities. Dubbed "death trains" or "phantom trains" by Russian state media, the network of covert boxcar-style launchers and missile complexes, initially designed by Soviet engineers, would boost the stealth, mobility, and counterstrike capability of Russia's arsenal.

The U.S. strategic arsenal, meanwhile, is undergoing a massive modernization that Defense Secretary Ash Carter has said will cost $350 billion, and outside experts say will in fact be closer to $1 trillion over 30 years. The U.S. House Appropriations Committee noted in the 2017 Department of Defense spending bill now making its way through Congress that that figure poses "an enormous affordability challenge."

Greg Thielmann, senior fellow at the Washington-based Arms Control Association, said Russia's deployment of the new Satan-2 -- its first flight is expected this summer -- was particularly troubling because silo-based missiles are considered more vulnerable to counterstrikes. Also, he said, Russia will need to deploy many more warheads in order to keep up their overall count as the older-generation Satan missiles are retired.

"There are a lot more things that need to be talked about on the margins," Thielmann said. "I'm much less worried about new Russian ICBMs in terms of keeping the overall balance stable, than I am about some of these things that are introducing new technologies into the strategic balance."

That includes armed drones or new hypersonic glide missiles being aggressively developed by the United States, along with Russia and China, he said.

"If both sides keep doubling down, you could see [things] kind of a spiral into an arms race that neither side really wants, but I see both sides somewhat reluctant to go too far in that direction," Gorenberg said. "At least for now."